This invention is directed to an ultrasonic data system and in particular to an ultrasonic air data system for aircraft such as helicopters.
Prior art systems in wide use on current aircraft suffer from well-known and often fundamental problems and limitations, particularly in the measurement of low airspeeds at variable direction. The response characteristics of pitot-static systems are, by nature, extremely poor below a certain threshold velocity, typically 40 knots. These systems are inherently non-linear with velocity and are also sensitive to air density. To derive true airspeed from such systems requires scaling for air density and other variables. Problems can also arise due to contamination of the pressure-sensing tubes and ports, which is generally present to a greater degree in the helicopter operating environment.
Other prior art systems, such as those relying on measurement of the passage frequency of Von Karmann vortices shed from a bluff body in the airstream yield no information, intrinsically, concerning the direction of the flow.
Prior art systems based on rotating total pressure sensors have moving parts which lead to unreliability. They can also foul up due to water, ice and dirt in the environment.
Systems based on thermometric effects (differential cooling) are subject to severe inaccuracies caused by environmental conditions such as rain and icing, due to the high latent heats of water. They are also inherently non-directional.
A number of the above systems are discussed by B. M. Elson in the article -- "Upgraded Helicopter Air Systems Urged" -- published in the Aviation Week and Space Technology periodical of July 26, 1976, pages 46-53. Ultrasonic systems have generally been developed for use in liquid flow measurements, however as discussed in U.S. Pat. No. 3,548,653 which issued on Dec. 22, 1970, as well as in the above article, some systems, having mutually transmitting pairs of transducers, have been adapted to measure velocity of an aircraft through air.